It is relatively frequent that persons older than forty years suffer from presbyopia, or farsightedness, meaning that his/her eyes lose the focus ability at different distances, and appearing especially in vision loss at short distances.
Customarily, presbyopic persons need several different corrections according to the sight distance. A type of lens providing said corrections is the bi-focal (or multi-focal) lens, which comprises at least two clearly differentiated sectors, an upper sector for far vision and a lower sector for near vision. One of the most uncomfortable features in the bifocal lenses for wearers is the visible line dividing said sectors.
In order to minimize this problem, progressive lenses have been developed in the last decades exhibiting a continuous correcting power range to see at all distances. This is achieved by continuously varying the radius of curvature of the lens surface.
In progressive lenses, contrary to what happens in bifocal lenses, there are neither visible lines nor sudden power changes, but these advantages are obtained at the expense of increasing aberrations in the lens periphery, since it is not possible to form a progressive surface utterly devoid of optical defects in its periphery.
The progressive lenses comprise four functionally different areas: an upper area for long distance, a lower area for short distance, a transition area therebetween, and a distorted vision peripheral area.
In recent years, advances in progressive surface geometry, in software of design thereof and in lens manufacturing technology have allowed to reduce the peripheral aberrations and bringing them close to their theoretical limits.
On the other hand, given the great number of design combinations the progressive lenses may assume and the variety of conditions to be met, there is not any univocally correct solution for all the wearers in all the situations of use of the lenses. Furthermore, modifying the size of one area affects the size of the others and it alters the level of aberration.
A recent tendency in the design of progressive lenses points to lens customisation to improve the wearer's response. This is currently feasible due to the steps forward in numerical control machining and computer aided manufacturing technology.
It is known that many wearers of progressive lenses designed under “industrial” criteria, not customised, have found difficulties in adaptation to their lenses. This is due to the fact that, with a conventional system, the manufacturer offers a limited series of lenses with some optical features averaged depending upon the prescribed optical correction and the main use of the lenses (reading, outdoor activities, etc). In this context, customisation involves incorporation of the wearer's personal features into the lens design process, and it may be applied to the design of any type of lens for spectacle frame, not only to the progressive lens design.
This customisation is based upon measuring by some means the individual's eye and head movement pattern. It has been proven that the coordination of the eye and the head movements is the result of an individual behaviour, a personal strategy of vision. All types of visual behaviours have been found, from people moving their eyes a lot and their head a little, to those moving their head a lot and their eyes a little.
For example, a presbyopic person predominantly moving his/her eyes for reading will be better adapted to progressive lenses which design contains a relatively wide short distance vision area and a relatively long transition area (smoother), whereas for one individual predominantly moving his/her head lenses having a relatively narrow short distance vision area an relatively short transition area (more abrupt) will be better.
EP0880046 teaches a method of manufacturing a progressive lens in which design information relating to the shape of the frame and the habits and activities of the individual has been included. This design is carried out by means of a computer program.
WO01/62139 relates to a method of designing a lens using a movement detection system for determining the movement of the head of lens wearer while performing a series of visual exercises. Then a computer, by virtue of a calculation algorithm, provides an inference in the movement of the wearer's eyes. The data obtained are combined with the wearer's personal information for classifying him/her into a visual behaviour category. Finally, a computer application, based on predetermined relationships, selects a progressive lens in keeping with the category of the wearers visual behaviour.